Motor vehicles with driven axle independent suspensions include a pair of axle shafts (also referred to as split axles or half shafts), one for each wheel, as described, merely by way of exemplification, in U.S. Pat. No. 4,699,235 issued on Oct. 13, 1987 to Anderson and assigned to the assignee of the present patent application, the disclosure of which is hereby incorporated herein by reference.
Referring now to FIG. 1A, the split axle drive system of U.S. Pat. No. 4,699,235 will be briefly described for point of reference, it being understood the present invention may apply to two wheel drive or four wheel drive systems.
Shown is a schematic plan view of a part-time four-wheel drive vehicle, comprising an internal combustion engine 10, transmission 12 and transfer case 14 mounted on a vehicle chassis (not shown). The engine 10 and transmission 12 are well-known components as is the transfer case 14 which typically has an input shaft (not shown), a main output shaft 16 and an auxiliary output shaft 18. The main output shaft 16 is drive connected to the input shaft in the transfer case 14 and is customarily aligned with it. The auxiliary output shaft 18 is drive connectable to the input shaft by a clutch or the like in the transfer case 14 and customarily offset from it. The transfer case clutch is actuated by a suitable selector mechanism (not shown) which is generally remotely controlled by the vehicle driver.
The main output shaft 16 is drivingly connected to a rear propeller shaft 20 which in turn is drivingly connected to a rear differential 22. The rear differential 22 drives the rear wheels 24 through split axle parts in a well-known manner. The auxiliary output shaft 18 is drivingly connected to a front propeller shaft 26 which in turn is drivingly connected to a split axle drive mechanism 28 for selectively driving the front wheels 30 through split axle parts. The split axle drive mechanism 28 is attached to the vehicle chassis by means including a bracket 34 on an extension tube 32.
Suitable split axle parts, commonly referred to as half shafts, are well-known from front wheel drive automobiles. These may be used for connecting the split axle drive mechanism 28 to the front wheels 30. The drawings schematically illustrate a common type of half shaft for driving connection to independently suspended steerable vehicle wheels comprising an axle shaft 40 having a plunging universal joint 42 at its inboard end adapted for connection to an output such as the flange 36 or 38 and the well-known Rzeppa-type universal joint 44 at its outboard end adapted to be connected to the vehicle wheel 30.
FIG. 1B depicts an example of a prior art motor vehicle rear suspension 52 of a motor vehicle drive system which incorporates a pair of axle shafts 50. The axle shafts 50 are in the form of a set of two symmetric axle shafts: a first axle shaft 50a and a second axle shaft 50b. The rear suspension 52 includes a cradle 54 which is attached, in this application, by resilient cradle mounts 56 to a frame (not shown) of the motor vehicle. A rear differential module 58 is connected to the cradle 54 via resilient rear differential module mounts 60, and is further connected, via constant velocity joints 62a, 62b to the first and second axles shafts 50a, 50b, respectively, of the axle shafts 50. The first and second axle shafts 50a, 50b are independently suspended via the constant velocity joints 62a, 62b so they are able to independently articulate along arrows 64a, 64b. A propeller shaft 66 is connected at one end to a transmission (not shown) and at its other end, via a constant velocity (or other type of) joint 68, to the rear differential module 58.
Problematically, axle shafts frequently exhibit “powerhop” when a large amount of torque is applied thereto. Powerhop typically occurs when tire friction with respect to a road surface is periodically exceeded by low frequency (i.e., below about 20 Hz) oscillations in torsional windup of the axle shafts. Powerhop produces oscillatory feedback to suspension and driveline components and can be felt by the vehicle occupants, who may describe the sensation as “bucking,” “banging,” “kicking” or “hopping.”
Axle shafts are typically manufactured from steel bar material and, as such, act as very efficient torsional springs. In the interest of reducing unwanted oscillations in the axle shafts, the standard practice has been to adjust the size (i.e., increasing the diameter) of the axle shafts in such a way to minimize the negative impact of oscillations by increasing the overall torsional stiffness of the axle shafts, thereby reducing powerhop. However, increasing the diameter of the axle shafts results in additional packaging, mass and cost related problems, while not really addressing the core issue of directly damping oscillations that are associated with powerhop, to with: lack of damping to absorb energy placed into the driveline by the negative damping characteristics of the tires during hard longitudinal acceleration or deceleration.
FIG. 1C is a graph 70 of axle shaft torque versus time for conventional symmetric axle shafts, wherein plots 72, 74 are respectively for each axle shaft, and wherein each axle shaft has a torsional stiffness of 525 Nm/deg. (i.e., Newton meters per degree). It will be seen that torque oscillations are in phase, whereby the conditions for powerhop are not mitigated in that the torque oscillations of each axle shaft are constructive with respect to each other.
Referring now to FIG. 1D, shown is a prior art solid axle rear drive system 78 which attempts to mitigate powerhop, wherein a forward torque arm 80 spans between the transmission 82 and the rear differential module 84. A pair of solid axle shafts 86a, 86b are operatively connected, without independent articulation, to the rear differential module 84. A track bar 87, a stabilizer 88, and a lower control arm 89 are further included.
While a forward torque arm may reduce powerhop in solid axle applications, as in FIG. 1D, it is not suitable for independently suspended axle shafts as in FIGS. 1A and 1B. This is demonstrated at FIGS. 1E and 1F. FIG. 1E is a graph 90 of axle shaft torque versus time for a symmetric axle shaft set, each axle shaft having a torsional stiffness of 525 Nm/deg., and the propeller shaft torsional stiffness is 138 Nm/deg. Plot 92 shows lack of damping, in fact the plot shows an increasing amplitude of the torque oscillations with increasing time. FIG. 1E is a graph 94 plotting vertical tire force, Plot 96, and plotting tire angular slip velocity, plot 98, both versus time for the drive system of FIG. 1E. It will be seen that Plots 96 and 98 are out of phase, wherein vertical tire force is increasing as tire angular slip velocity is decreasing. Therefore, because a forward torque arm provides a negative coupling as powerhop decays, it is unable to mitigate powerhop for independently suspended, driven axle shafts; indeed, the negative coupling may actually promote powerhop oscillations. In this regard, by “negative coupling” is meant vertical tire force and wheel angular velocity are out of phase, even opposite phase; whereas by “positive coupling” is meant vertical tire force and wheel angular velocity have similar phase. Therefore, a forward torque arm is not suitable for mitigating powerhop for independently suspended, driven axle shafts.
Accordingly, there is a clearly felt need for damping of independently suspended axle shafts so as to thereby provide reduction of powerhop and associated driveline disturbances, such as for example axle shutter.